The speed sensors typically used for agricultural monitors and control systems fall into two categories: magnetic and radar. Magnetic sensors, typically mounted on the wheel or drive shaft of the agricultural vehicle, may produce inaccurate speed readings due to wheel slippage, wheel distortion and other factors. Radar sensors, while calculating the speed of the vehicle independently of wheel motion, do not discriminate between forward and reverse motion and are typically less accurate at slower speeds. Radar sensors may also produce measurement errors due to equipment vibration or signal interference caused by reflections from field debris or waving weeds. With respect to either magnetic or radar sensors, the incorrect speed readings which may arise can create significant errors in distance and area measurement, yield calculations and chemical application.
Doppler-based speed sensing systems are in use which determine velocity based on the received and transmitted frequency signals. For example, U.S. Pat. No. 4,728,954 discloses a Doppler-based speed sensor approximating ground velocity with the equation: ##EQU1## where a is the angle of the signal transmission access from horizontal, C is the speed of sound, F(t) is the transmitted signal frequency, and F(r) is the received signal frequency.
Existing ultrasonic systems typically use multiple parts for transmitting and receiving the signal to be reflected. Existing system are also susceptible to inaccurate readings which arise from the change in the speed of sound due to changes in temperature and due to erroneous readings caused by ground interference and signal interference. Accuracy is also reduced by the approximate nature of the above-described equation Finally, existing sensor outputs do not distinguish between forward and reverse vehicle directions.